#include<bits/stdc++.h>
using namespace std;
int main()
{
	int a,b,c;
	float ans;
	cin>>a>>b>>c;
	if((a*a)+(b*b)==(c*c))
		ans=a*b/2;
	else if((a*a)+(c*c)==(b*b))
		ans=a*c/2;
	else if((c*c)+(b*b)==(a*a))
		ans=c*b/2;
	else
	{
		cout<<"No";
		return 0;
	}
	printf("%.1f",ans);
	return 0;
} 
